Dynamic mechanical polymer nanocomposites

ABSTRACT

Polymer nanocomposites exhibit a reversible change in stiffness and strength in response to a stimulus. The polymer nanocomposites include a matrix polymer with a comparably low modulus and strength and nanoparticles that have a comparably high modulus and strength. The particle-particle interactions are switched by the stimulus, to change the overall material&#39;s mechanical properties. In a preferred embodiment, a chemical regulator is used to facilitate changes of the mechanical properties. Methods for inducing modulus changes in polymer nanocomposites are also disclosed.

CROSS REFERENCE

This application claims the benefit of priority under 35 U.S.C. §119 ofU.S. Provisional Application Ser. No. 61/123,372 filed on Apr. 8, 2008.

FIELD OF THE INVENTION

The present invention relates to polymer nanocomposites that exhibit areversible change in stiffness and strength in response to a stimulus.The polymer nanocomposites include a matrix polymer with a comparablylow modulus and strength and nanoparticles that have a comparably highmodulus and strength. According to the invention, the particle-particleinteractions are switched by the stimulus, to change the overallmaterial's mechanical properties. In a preferred embodiment, a chemicalregulator is used to facilitate changes of the mechanical properties.Methods for inducing modulus changes in polymer nanocomposites are alsodisclosed.

BACKGROUND OF THE INVENTION

Many echinoderms, such as sea cucumbers, share the ability to rapidlyand reversibly alter the stiffness of their connective tissue. In thecase of sea cucumbers, this morphing occurs within seconds and createsconsiderable survival advantages. A series of recent studies on thedermis of these invertebrates has provided evidence that this defensemechanism is enabled by a nanocomposite structure in which rigid,high-aspect ratio collagen fibrils reinforce a viscoelastic matrix offibrillin microfibrils. The stiffness of the tissue is regulated bycontrolling the stress transfer between adjacent collagen fibrilsthrough transiently established interactions. These interactions aremodulated by soluble macromolecules that are secreted locally byneurally controlled effector cells. The dermis of the Cucumaria frondosaand other sea cucumber species represents a compelling model of a chemoresponsive material in which a modulus contrast by a factor of 10 (˜5 to˜50 MPa) is possible.

In view of the above, it would be desirable to provide artificialdynamic materials that exhibit stimuli-responsive mechanical propertiessimilar to the abilities displayed by many echinoderms, especiallymaterials that could be used in biomedical applications.

SUMMARY OF THE INVENTION

The present invention discloses polymer nanocomposites that exhibitreversible modulus and strength switching in response to stimuli such asa chemical, thermal and/or electrical stimulus. Relatively, large ordersof modulus contrast can be obtained utilizing the systems disclosed bythe present invention.

Methods of inducing modulus switching in polymer nanocomposites are alsodisclosed, wherein the modulus contrast is a factor greater than 2.5. Awide range of nanoparticles can be used to induce modulus changes in anarray of matrix polymers.

Accordingly, it is an objective of the present invention to providestimuli-responsive polymer nanocomposites that exhibit reversiblemodulus changes.

It is a further objective of the present invention to provide methodsfor inducing modulus changes in polymer nanocomposites. The methodincorporates a stimuli such as chemical or electrical stimuli.

In one aspect of the present invention, a method for inducing a moduluschange in a polymer nanocomposite is disclosed, comprising the steps ofproviding a polymer nanocomposite comprising a nanoparticle networkincorporated into a host matrix polymer, wherein the nanoparticlenetwork is a substantially continuous three-dimensional network ofsubstantially dispersed nanoparticles that exhibit at least someinteractions among each other; and inducing a modulus change in thepolymer nanocomposite by exposing the polymer nanocomposite to astimulus that reduces interactions among the nanoparticles, wherein themodulus change exhibited is by a factor greater than 2.5.

In another aspect of the present invention, a method for inducing amodulus change in a polymer nanocomposite is disclosed, comprising thesteps of providing a polymer nanocomposite comprising a nanoparticlenetwork incorporated into a host matrix polymer, wherein thenanoparticle network is a substantially continuous three-dimensionalnetwork of substantially dispersed nanoparticles; and inducing a moduluschange in the polymer nanocomposite by exposing the polymernanocomposite to at least a combination of chemical and temperaturestimuli that reduce nanoparticle self interactions, wherein the moduluschange exhibited is by a factor greater than 2.5.

In still a further aspect of the present invention, a polymernanocomposite is disclosed, comprising a nanoparticle networkincorporated into a host matrix polymer, wherein the nanoparticlenetwork is a formation of a substantially three-dimensional network ofsubstantially dispersed nanoparticles, wherein in a first switchedstate, the composite has a first modulus, and wherein in a secondunswitched state, the composite has a second modulus, wherein the firstmodulus is greater than the second modulus by a factor greater than 2.5.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and other features andadvantages will become apparent by reading the detailed description ofthe invention, taken together with the drawings, wherein:

FIG. 1 is a transmission electron microscopy image of tunicate whiskersisolated from tunicate mantles (scale bar=1 μm);

FIG. 2A graphically illustrates representative DMA traces that showtensile storage moduli E═_(c) of dry EO-EPI and EO-EPI/whiskernanocomposites as a function of whisker content and temperature;

FIG. 2B graphically illustrates representative DMA traces that show losstangents tan δ of dry EO-EPI and EO-EPI/whisker nanocomposites as afunction of whisker content and temperature;

FIG. 3 graphically illustrates representative DMA traces that showtensile storage moduli E′_(c) of EO-EPI and EO-EPI/whiskernanocomposites as a function of whisker content and temperature, andwherein the samples were equilibrated by immersion for 48 hours indeionized water and were measured under submersion in deionized water;

FIG. 4 illustrates tensile storage moduli E′_(c) of EO-EPI andEO-EPI/whisker nanocomposites as a function of whisker content, whereinlines represent values predicted by the percolation and Halpin-Kardosmodel for dry samples, and wherein data points represent averages(number of individual measurements, N, =3-5)±standard errormeasurements;

FIG. 5 illustrates representative stress-strain curves of neat EO-EPIand EO-EPI/whisker nanocomposites containing 14.3% v/v whiskers, whereinthe materials were conditioned by either drying in vacuum or equilibriumswelling in deionized water;

FIG. 6 illustrates tensile storage moduli E′_(c) of EO-EPI/whiskernano-composites as a function of volume fraction of cellulose whiskers,wherein the nanocomposites were conditioned by either drying in vacuum,equilibrium swelling in deionized water, or swelling to saturation indeionized water followed by a re-drying in vacuum, wherein the data weretaken from FIG. 2A, but data for the swollen samples were plotted fororiginal whisker content to allow for a direct comparison of the E′_(c)of dry and water-swollen composites for the same composition, andwherein data points represent averages (N=3-5)±standard errormeasurements;

FIG. 7 illustrates systematic variations made to individual parametersfed into the percolation model, wherein lines represent (i) the modelused to fit vacuum-dried nano-composites in FIG. 2A, (ii) same as (i)but with E′_(s)=0; (iii) same as (i) but with E′_(r)×1000; (iv) same as(i) but with E′_(r)/1000, wherein setting E′_(s) to zero represents acomplete failure in the mechanical integrity of the matrix polymer; andwherein variations in E′_(r) are made to investigate effects ofstrengthening (1000 times), or weakening ( 1/1000 times) thewhisker-whisker interactions;

FIG. 8A illustrates loss tangents tan δ of dry PVAc and PVAc/whiskernanocomposites as a function of whisker content and temperature;

FIG. 8B illustrates tensile storage moduli E′_(c) of PVAc andPVAc/whisker nanocomposites (dry and water swollen) as a function ofwhisker content, wherein lines represent values predicted by thepercolation and Halpin-Kardos model for dry and water-swollen samples,respectively; wherein data points represent averages (number ofindividual measurements, N≧2); and wherein water swollen samples withhigher whisker content display decreased moduli below the Halpin-Kardosmodel, most likely due to the increased swelling at high whiskercontent;

FIG. 9 illustrates solvent uptake as a function of whisker volumefraction and temperature upon immersion (to equilibration) ion deionizedwater or ACSF; wherein data points represent averages (N=4-5)±standarderror measurements;

FIG. 10A illustrates representative DMA traces that show tensile storagemoduli E′_(c) of water-swollen PVAc and PVAc/whisker nanocomposites as afunction of whisker content and temperature;

FIG. 10B illustrates representative DMA traces that show loss tangentstan δ of water-swollen PVAc and PVAc/whisker nanocomposites as afunction of whisker content and temperature;

FIG. 11A illustrates tensile storage moduli E′ of EO-EPI/whiskernano-composites as a function of volume fraction of cellulose whiskers,wherein the nanocomposites were conditioned by either drying in vacuum,equilibrium swelling in deionized water, or swelling to saturation indeionized water followed by redrying in vacuum; wherein lines representvalues predicted by the percolation and Halpin-Kardos model; wherein thearrow indicates changes in modulus and volume fraction of whiskersresulting from aqueous swelling of one selected sample (19% v/vwhiskers);

FIG. 11B illustrates solvent uptake as a function of whisker volumefraction under ambient conditions, immersion in deionized water, orisopropanol at room temperature;

FIG. 11C illustrates tensile storage moduli E′ of IPA-swollenEO-EPI/whisker nanocomposites as a function of volume fraction ofcellulose whiskers; wherein lines represent values predicted by thepercolation and Halpin-Kardos model, and wherein data points representaverages (number of individual measurements, N=3-6)±standard errormeasurements; and

FIG. 12 illustrates tensile storage moduli E′_(c) of PBMA andPBMA/whisker nanocomposites (dry and water swollen) as a function ofwhisker content, wherein lines represent values predicted by thepercolation and Halpin-Kardos model for dry and water-swollen samples,respectively; wherein data points represent averages (number ofindividual measurements, N≧2); and wherein ACSF swollen samples withhigher whisker content display decreased moduli closer to theHalpin-Kardos model, most likely due to the increased swelling at highwhisker content.

DETAILED DESCRIPTION OF THE INVENTION

The variable modulus polymer nanocomposites are derived from a matrixpolymer and nanoparticles dispersed therein. Achieving a balance ofattractive and repulsive interactions between the nanoparticles is a keyfor producing polymer nanocomposites exhibiting desired dynamicmechanical properties.

Numerous different types of nanoparticles can be utilized in the presentinvention. Generally any nano-size particles can be utilized. In oneembodiment, nanoparticles that can disperse substantially fully in atleast one solvent system are preferred. Nanoparticles must haveparticle-particle interactions which include, but are not limited to,hydrogen bonding, ionic charges, hydrophobic interactions, or pi-pistacking. For example, nanoparticles suitable for use in the presentinvention include, but are not limited to, nanofibers, for examplecellulose-based whiskers; nanotubes such as carbon nanotubes andnano-size platelet materials, such as certain clays, or a combinationthereof. In a preferred embodiment, the nanoparticles have a relativelyhigh aspect ratio−length L/diameter d (L/d), of about 5 or more,preferably 10 or more, more preferably 20 or more, and most preferably50 or more. Nanofibers are preferred nanoparticles in one embodiment.

To create the stimulus-responsive nanocomposites according to thepresent invention, a substantially continuous three-dimensional networkof substantially dispersed nanoparticles that exhibit at least someinteractions among each other, preferably a dynamic nanofiber network ofthe nanoparticles, is formed in the host polymer, preferably through asolution casting or a sol/gel process.

For example, in the case of tunicate whiskers, this involves theformation of a homogeneous whisker dispersion in a medium, such aswater, such as taught by M. M. de Souza Lima, R. Borsali R., Macromol.Rapid Commun. 25, 771 (2004); M. A. S. A. Samir, F. Alloin, A. Dufresne,Biomacromolecules 6, 612 (2005); R. H. Marchessault, F. F. Morehead, N.M. Walter, Nature 184, 632 (1959); A. Sturcova, J. R. Davies, S. J.Eichhorn, Biomacromolecules 6, 1055 (2005); and O. van den Berg, J. R.Capadona, C. Weder, Biomacromolecules 8, 1353 (2007), hereinincorporated by reference. Sonication or other methods of dispersionsuch as stirring or high shear mixing can be used to disperse thenanoparticles in a preferred embodiment.

One or more of the water and solvents utilized to disperse thenanoparticles can include additional components such as, but not limitedto, various additives such as stabilizers, monomers, polymers,surfactants, etc. As indicated herein, biorenewable nanoparticles, suchas the cellulose-based whiskers, are preferred in one embodiment, andare available from a number of sources, such as wood, cotton and variousanimals such as tunicates. Tunicate whiskers exhibit high stiffness,tensile modulus about 143 GPa and dimensions at the nanometer scale, forexample 26 nm by 2.2 μm, see FIG. 1. Similar nanofibers can be obtainedfrom a number of renewable biosources including wood and cotton.

Tunicate whiskers are desirable for use as their aspect ratio isrelatively high, which is advantageous for the formation of percolatingarchitectures. Because of the high density of strongly interactingsurface hydroxyl groups, cellulose whiskers have a strong tendency foraggregation. The whisker-whisker interactions can be moderated by theintroduction of sulfate surface groups, which promote dispersability inselect hydrogen-bond-forming solvents. This balance of attractive andrepulsive interactions is an important factor for the fabrication ofcellulose-whisker nano-composites. Good dispersion is achieved duringprocessing when whisker self-interactions are “switched off” bycompetitive binding with a hydrogen-bond-forming solvent. Uponevaporation of the solvent, the interactions among the whiskers are“switched on” and they assemble into a percolating network. Thisarchitecture and strong interactions among the whiskers maximize stresstransfer and therewith the overall modulus of the nanocomposite.

One preferred method to fabricate the stimulus-responsivenano-composites according to the present invention is a templateapproach such as taught by J. R. Capadona, O. van den Berg, L. Capadona,D. Tyler, S. Rowan, and C. Weder, Nature Nanotechnology 2, 765 (2007),herein incorporated by reference. Following whisker dispersion in amedium such as water, as described above, a whisker gel is formedthrough solvent exchange with a solvent that is medium-miscible, such aswater-miscible, but does not disperse the whiskers. Variouswater-miscible solvents known in the art can be utilized including, butnot limited to, acetone, methanol, tetrahydrofuran, ethanol,acetonitrile, dioxane and isopropanol, or a combination thereof.

The whisker content of the gels can be controlled over a broad range bythe concentration of the initial whisker dispersion, the volume ratio oforganic solvent to whisker dispersion, wherein more solvent leads tolower whisker content, and the nature of the solvent, i.e., thesolvation energy thereof. Within the framework of the template approach,one or more polymers or copolymers utilized to form the polymernanocomposites are dissolved in an appropriate solvent. The solvent isselected so that it does not substantially re-disperse the nanoparticlesnetwork in the gel described. Many different polymers and copolymers canbe utilized as a host polymer. Examples of suitable (co)polymers, i.e.,polymers or copolymers, include, but are not limited to, variousalkylene oxide polymers and copolymers such as ethylene oxide, propyleneoxide, copolymers of ethylene oxide and epichlorohydrin and/or othermonomers; a vinyl aromatic (co)polymer such as polystyrene and styrenecopolymers; polyolefin polymers or copolymers such as polyethylene andpolypropylene; diene polymers and copolymers, such as cis-polybutadiene;polyacrylates and acrylate copolymers, such as methyl methacrylate;polyamides; and polyester polymers or copolymers such as poly(vinylacetate) or polycaprolactone.

Within the framework of the template approach, the nanoparticle gel isimbibed with the polymer solution. The resulting nanocomposite is driedand, if desired, processed and/or shaped further.

Another preferred method to fabricate the stimulus-responsivenano-composites according to the present invention is casting from acommon solvent such as taught by O. van den Berg, M. Schroeter, J. R.Capadona, and C. Weder, J. Mater. Chem. 17, 2746 (2007), hereinincorporated by reference. While the nanoparticles are dispersed in amedium include such as, for example, but not limited to, water,N,N-dimethyl formamide, dimethyl sulfoxide, N-methyl pyrrolidone, formicacid, m-cresol, or a combination thereof, as described above one or moredesired polymer or copolymers are dissolved in a suitable solvent. Theamounts of polymer incorporated into the solvent can vary as desired.The polymer solvent can comprise auxiliary components if desired, suchas, but not limited to, stabilizers, surfactants, etc., if desired.Compatible polymer solvents are known to those of ordinary skill in theart.

In a preferred embodiment of the invention, the polymer is dissolved inthe same solvent or combination of solvents as is used to disperse thenanoparticles. Examples of suitable polymer solvents include, forexample, but are not limited to, water, N,N-dimethyl formamide, dimethylsulfoxide, N-methyl pyrrolidone, formic acid, m-cresol, or a combinationthereof. The range of polymer in the solvent in one embodiment can befrom about 1 to about 40% w/w, but this dependent on the viscosity ofthe polymer solution and it is to be understood that the amounts can behigher or lower depending on the polymer and solvent utilized.

Polymer nanocomposites are prepared by combining the desired amounts ofthe colloidal nanoparticles dispersion and polymer solution, andsolution casting the resulting preferably homogenous mixture. In oneembodiment, the solution in a desired vessel is placed into a vacuumoven to evaporate the solvent and dry the resulting film. In oneembodiment, the vacuum oven has a temperature of 60° C., pressure of 15mbar, Suitable pressures and drying times will vary depending upon thesystem utilized.

In one embodiment, the resulting polymer nanocomposites are shaped afterthe fabrication by, for example, the template approach or casting from acommon solvent; this can be achieved, for example by compression moldingat a desired pressure for a predetermined period of time, or any othermelt-process that is known to those skilled in the art. For example,when the polymer chosen is an ethylene oxide-epichlorohydrin copolymer(EO-EPI), the polymer nanocomposite can be compression molded at atemperature of about 80° C. at about 600 psi for about two minutes. In afurther embodiment, when the polymer is polyvinyl acetate (PVAc),compression molding can be performed at about 90° C. at 0 psi for abouttwo minutes, followed by an increase in pressure to 3,000 psi for about15 minutes. Compression molding is utilized in one embodiment to yieldnanocomposite films, such as about 20 to about 500 micrometers.

It is desirable to have a concentration of nanoparticles in the hostmatrix polymer, such that the nanoparticles form a substantiallypercolating network in a stiff or switched state. As known to those ofordinary skill in the art, the percolation concentration can beestimated from the aspect ratio of the nanoparticles utilizing standardpercolation models. For example, in one embodiment, the nanoparticles,such as tunicate whisker nanoparticles, may have a concentration fromabout 3% to about 40%, and desirably from about 5% to about 30% basedupon the total volume of the host matrix polymer. When generally shorterfillers are utilized and the aspect ratio is, therefore, lower, minimumconcentration of the nanoparticles in the host matrix polymer isgenerally higher.

It has been surprisingly found that the key for fabricatingnanocomposites that display switchable mechanical properties accordingto the present invention is to (1) select nanoparticles that can displaydesirable particle-particle interactions; (2) utilize a fabricationprocess, as described hereinabove, that leads to materials in which saidnanoparticles substantially form a percolating network in said polymermatrix; (3) utilize a fabrication process that prevents significantphase separation of the nanoparticles; and (4) employ a polymer matrixthat is able to let the stimulus switch on and off saidparticle-particle interactions.

For example, when nanocomposites of cellulose whiskers and EO-EPI, notaccording to the invention, were prepared by a prior art method astaught by M. Schroers, A. Kokil, and C. Weder, J. Appl. Polym. Sci. 93,2883 (2004), materials were obtained that display a comparatively weakreinforcement through introduction of the whiskers. The reinforcement ismuch lower than predicted by the percolation model, which is describedelsewhere in this application. This discrepancy is the result of phaseseparation of whiskers and polymer. As a result, these prior artmaterials do not exhibit an appreciable change of their mechanicalproperties if immersed in water. This is in stark contrast to materialsof nominally similar composition, but which were prepared according tothe present invention.

Fabrication of cellulose whisker nanocomposites according to the presentinvention. Lyophilized cellulose whiskers, prepared for example astaught by J. R. Capadona, O. van den Berg, L. Capadona, D. Tyler, S.Rowan, and C. Weder, Nature Nanotechnology 2, 765 (2007), hereinincorporated by reference, were dispersed in dimethyl formamide (DMF) ata concentration of 5 mg/mL. The EO-EPI copolymer or PVAc polymer wasdissolved in DMF (5% w/w) by stirring for two days. Nanocomposites wereprepared by combining the desired amounts (to yield materials containing0.8%-19% v/v whiskers) of the colloidal whisker dispersion and polymersolution, and solution-casting the resulting homogeneous mixture intoTeflon® Petri dishes. The dishes were placed into a vacuum oven (60° C.,15 mbar, EO-EPI=48 hrs.; PVAc=1 week) to evaporate the solvent and drythe resulting films, before the material was compression-molded betweenspacers in a carver laboratory press (EO-EPI=80° C. at 6000 psi for 2min.; PVAc=90° C. at 0 psi for 2 min., followed by an increase ofpressure to 3000 psi for 15 min.) to yield 50-500 μm thin nanocompositefilms.

The thermo-mechanical properties of the polymer nanocomposite materialscan be established by dynamic mechanical analysis and tensile tests. Ithas been found that the mechanical contrast with respect to the tensilestorage modulus is produced by swelling the polymer nanocomposite in asuitable solvent, such as those described hereinabove. The dry polymernanocomposite exhibits the greatest tensile storage modulus, whereasequilibrium swelled polymer nano-composites have a greatly reducedtensile storage modulus. As indicated hereinabove, the nanoparticles are“switched on” and assembled in a substantially percolating network inthe absence of a solvent. Upon the addition of a solvent, preferably ahydrogen bond forming solvent, to the polymer nanocomposite, the whiskerself interactions are “switched off” by competitive binding. In oneembodiment, from about 10% w/w to about 85% w/w solvent based on thetotal weight of the composition is utilized. Relatively low amounts ofsolvent are preferred. Utilizing the switching techniques of the presentinvention, desirable changes in modulus can be achieved. For example,between a first switched state and a second switched state, the moduluscontrast is a factor of generally greater than 2.5, desirably 5 or more,and preferably 10 or more, and most preferably 20 or more. Examples ofswitching polymer nanocomposites follow.

Switching Experiments with EO-EPI/cellulose whisker nanocomposites.Compression molded EO-EPI/whisker nanocomposites were dried in a vacuumoven (60° C., 15 mbar, 48 hours) to remove all water and stored in adesiccator until dynamic mechanical analysis (DMA) measurements weremade. For switching experiments according to invention vacuum-driedEO-EPI/whisker nanocomposites were placed into sealed vials filled withdeionized water, artificial cerebral spinal fluid (ACSF) for 48 hours.The extent of swelling was determined gravimetrically from the originalmass of the dry sample and the mass after swelling. Swollen samples werethen either measured by DMA using a submersion chamber filled withdeionized water or ACSF, or re-dried in vacuum (60° C., 15 mbar, 48hours) and measured by DMA to explore the reversibility of mechanicallyswitching.

Switching Experiments with PVAc/whisker Nanocomposites. Compressionmolded PVAc/whisker nanocomposites were dried in a vacuum oven (60° C.,15 mbar, 48 hours) to remove all water and stored in a desiccator untilDMA measurements were made. For switching experiments, vacuum-driedPVAc/whisker nanocomposites were placed into DMA using a submersionchamber filled with artificial cerebral spinal fluid (ACSF) at roomtemperature. The ACSF bath was heated at a nominal rate of 2° C./min toa temperature of 37° C., where the sample was held. DMA measurementswere made throughout this process. ACSF was prepared based on productinformation (Alzet Cupertino, Calif.).

Thermo Mechanical Testing. DMA temperature sweeps under oscillatorystress were performed on rectangular films of the neat polymers or thenano-composites using a TA instruments DMA Q800 in tensile mode with anoscillation frequency of 1 Hz, a static force of 10 mN, an oscillationamplitude of 15.0 μm, and an automatic tension setting of 125%.Measurements were carried out at a heating rate of 3° /min (range of 15°C.-45° C. for EO-EPI nanocomposites). Swollen samples were measuredusing a submersion clamp, filled with the appropriate medium.

Stress-strain experiments were performed at room temperature onrectangular films of the neat polymers or the nanocomposites using a TAinstruments DMA Q800 in constant strain mode with a strain rate of 2/minfor the nanocomposites or 2 or 20%/min for the neat polymers an initialamplitude of 15.0 (dry samples) or 150 μm (swollen samples). Swollennanocomposites were measured using a submersion clamp, filled withdeionized water.

The thermo-mechanical properties of EO-EPI/whisker nanocomposites with awhisker content between 0 and 19% v/v were established by DMA andtensile tests. DMA temperature sweeps (FIGS. 2A, 2B and 3) display aglass transition temperature (T_(g)) around −37° C. (maximum of losstangent, tan δ), which is independent on the whisker content and matchesthe T_(g) of the neat EO-EPI matrix (FIGS. 2A, 2B). The intensity of tanδ decreases more than proportionally with the whisker concentration(FIG. 2B), which is indicative of attractive polymer-whiskerinteractions. FIG. 11A shows the tensile storage moduli (E′_(c)) of dryEO-EPI/whisker nanocomposites extracted from the DMA traces for atemperature of 25° C., i.e. in the rubbery regime far above T_(g).E═_(c) increased with the whisker content from ˜3.7 MPa (neat polymer)to ˜800 MPa (19% v/v whiskers). The observed reinforcement confirms theformation of a percolating nanofiber network in which stress transfer isfacilitated by hydrogen-bonding between the whiskers. This hypothesis issupported by calculations obtained using a percolation model. Within theframework of the model, the tensile modulus of the nanocomposites(E′_(c)) can be expressed as:

$\begin{matrix}{E_{c}^{\prime} = \frac{{\left( {1 - {2\; \psi} + {\psi \; X_{r}}} \right)E_{s}^{\prime}E_{r}^{\prime}} + {\left( {1 - X_{r}} \right)\psi \; E_{r}^{\prime 2}}}{{\left( {1 - X_{r}} \right)E_{r}^{\prime}} + {\left( {X_{r} - \psi} \right)E_{s}^{\prime}}}} & (1) \\{{{with}\mspace{14mu} \psi} = {X_{r}\left( \frac{X_{r} - X_{c}}{1 - X_{c}} \right)}^{0.4}} & (2)\end{matrix}$

where E′_(s) and E′_(r) are the experimentally determined tensilestorage moduli of the neat EO-EPI (3.7 MPa) and a neat tunicate whiskerfilm (4.0 GPa), respectively, ψ is the volume fraction of whiskers thatparticipate in the load transfer, X_(r) is the volume fraction ofwhiskers, and X_(c) is the critical whisker percolation volume fractioncalculated by 0.7/A. A is the aspect ratio of the whiskers and has avalue of 84 as determined by analysis of transmission electronmicroscope images, see FIG. 1.

FIG. 11A shows that the experimentally determined E′_(c) values of dryEO-EPI/whisker nanocomposites agree with values obtained fromEquation 1. By contrast, the data deviate strongly from theHalpin-Kardos model (FIG. 4, vide infra). This behavior is indicativefor the formation of a percolating network of strongly interactingcellulose whiskers within the EO-EPI matrix. This architecture isconfirmed by atomic force microscopy (AFM) and scanning electronmicroscopy (SEM) images, which both show that the cellulose whiskersform a percolating network within the EO-EPI matrix. Stress straincurves (FIG. 5) reveal that the formation of a percolating network ofcellulose whiskers within the EO-EPI matrix not only affects E′_(c), butalso has a significant influence on the maximum tensile strength (a),which increased from 0.27±0.04 (neat EO-EPI, stress at break) to1.71±0.23 MPa (14.3% v/v whiskers, stress at yield), while theelongation at break was reduced from 360±20 to 6.7±0.8%, see Table 1.

TABLE 1 Whisker Sample Stress Stress Content Condi- at yield at BreakElongation at (% v/v) tion point (MPa) (MPa) Break (%) 14.3 (N = 7) Dry1.71 +/− 0.23 0.05 +/− 0.02 6.7 +/− 0.8 14.3 (N = 5) Swollen 0.37 +/−0.11 0.29 +/− 0.7  17.8 +/− 3.9    0 (N = 2) Dry Not Applicable 0.27 +/−0.04 360 +/− 20    0 (N = 2) Swollen Not Applicable 0.34 +/− 0.04 263+/− 100

To demonstrate that water could act as a chemical regulator for thewhisker-whisker interactions in the EO-EPI/whisker nanocomposites, themechanical properties of these materials were tested as a function ofwater exposure. The atmospheric water uptake of the materials isnegligible under ambient conditions; i.e., if not placed in an aqueousmedium (FIG. 11B). Dry EO-EPI/whisker nanocomposites were immersed indeionized water for 48 hours to achieve equilibrium swelling (FIG. 11B).Under these conditions, all compositions investigated exhibit modestaqueous swelling (ca. 30% v/v), indicating that in case of thesecompositions the water uptake is mainly governed by the matrix polymerwith only minor variations due to whisker content. The tensile storagemoduli for water-swollen EO-EPI/whisker nanocomposites were measured byDMA at 25° C. in deionized water. A very significant reduction of E′_(c)compared to the dry nanocomposites can be observed (FIG. 11A). Thegreatest mechanical contrast is seen in the case of the nanocompositewith the highest whisker content (nominally 19% v/v), where E′_(c) wasreduced from ˜800 to 20 MPa upon equilibrium swelling. At the same time,swelling with water leads to a significant decrease of the tensilestrength (1.71±0.23 to 0.37±0.11 MPa for a 14.3% v/v whiskernanocomposite, FIG. 5, Table 1 and an increase of the elongation atbreak (6.7±0.8 to 17.8±0.39%). Control experiments with the neat EO/EPI(FIG. 5, Table 1) show minimal changes in tensile strength upondeionized water swelling.

One argument that could be made against the interpretation that theobserved changes in modulus, elongation at break, and tensile strengthare the result of switching off the nanofiber-nanofiber interactions isthat simple swelling of the matrix alone could lead to a plasticizingeffect; however, careful analysis of our data shows that this is not thecase. DMA traces (FIGS. 2A & 2B) indicate that the EO-EPI/whiskernano-composites do not undergo any phase transition that would lead to adrop in modulus, such as cross-linked polymer hydrogels and hygroscopicpolymers, which can display a decrease of the glass transitiontemperature upon water uptake. While E′_(s) of the neat EO-EPI isreduced from 3.7 to 0.8 MPa upon equilibrium swelling with water (FIG.11A), analysis in the context of the percolation model (Equations 1-2,FIG. 7) shows that a reduction of E′_(s) alone cannot account for asignificant reduction of E′_(c). FIG. 11A also reveals that even aftercorrecting X_(r) for water uptake, the percolation model no longeradequately describes E′_(c) of the water-swollen nanocomposites. Bycontrast, the moduli now are in much closer agreement with theHalpin-Kardos model, which has successfully been used to describe themodulus of nanocomposites in which the filler is homogeneously dispersedin a polymer matrix and does not display pronounced filler-fillerinteractions. The model assumes that the materials are equivalent tomany layers of unidirectional plies oriented in alternating directions(−45°, 0°, 45°, and 90°) and the properties of the unidirectionalreference ply are predicted by the Halpin-Tsaï equations where themodulus in the longitudinal (E_(L)) and transverse (E_(T)) directs aregiven by:

E _(L) =E _(m)(1+2(A)η_(L)φ_(w))/(1−η_(L)φ_(w))   (3) and

E _(T) =E _(m)(1+2η_(T)φ₂)/(1−η_(T)φ₂)   (4)

Thus, all data indicate that the stiffness reduction achieved in theEO-EPI/whisker nanocomposites is related to the decoupling of thestress-transferring rigid nanofiber network upon introduction of wateras a competitive hydrogen-bonding agent. Consistent with the proposedmechanism, the switching is fully reversible: the materials adaptedtheir original stiffness upon drying (FIG. 11A).

To demonstrate specificity of the switching mechanism, isopropanol (IPA)was used as the swelling agent. IPA was selected because it swells neatEO-EPI to a similar degree as water (FIG. 11B), but is incapable ofdispersing cellulose whiskers. The nanocomposites swelled upon immersionin IPA (FIG. 11B) to a level similar to that of the composites in water;however E′_(c) barely changed in comparison to the dry state (FIG. 11C)and the data fit the percolation model. This result confirms that thechemo-mechanical response is largely a result of disruption of thewhisker-whisker interactions and not just simply plasticization of thematerial. By contrast, EO-EPI is plasticized considerably upon IPAswelling (E′_(c) drops from 3.6 to 0.93 MPa). This contrast highlightsthe most important advantage of the nanocomposite approach over simpleplasticization of a neat polymer. While plasticization through solventuptake, which is inherent to the latter, is a non-specific process, theresponsive nanocomposites can be designed to display a response that isspecific to the nature of the stimulus. In addition, the nanocompositeapproach provides the ability to increase the initial stiffness andstrength of the material and allows for the use of host polymers thathave no thermal transition in the temperature regime of interest, suchas the EO-EPI matrix used here.

To further maximize the dynamic range in which the mechanical propertiesof the nanocomposites according to the present invention can beswitched, we sought to combine the switching mechanism with a chemicallyinfluenced thermal transition. We surprisingly discovered thatnanocomposites based on poly(vinyl acetate) (PVAc) and cellulosewhiskers display such a “dual” responsive behavior. Our data show thatupon exposure to physiological conditions the materials undergo a phasetransition; in addition, the reinforcing whisker network isdisassembled. DMA experiments (FIG. 8) reveal that the neat PVAcdisplays a T_(g) around 42° C.; i.e., just above physiologicaltemperature. E′ of the neat polymer is considerably reduced upon heatingfrom room temperature (1.8 GPa at 23° C.) to above T_(g) (0.39 MPa at56° C.; this corresponds to T_(g)+16° C. and marks the temperature atwhich E′ is starting to level off). As evidenced by DMA data, theintroduction of cellulose whiskers into PVAc has only a minimalinfluence on T_(g) in the dry state (FIG. 8). The thermal transition issharpened and the temperature at which E′_(c) begins to drop isincreased from about 25 to over 40° C. For certain biomedicalapplications, this effect is very desirable, as it prevents thethermally-induced softening of the material just upon exposure to bodytemperature. As a consequence of the already rather high stiffness ofthe glassy PVAc matrix, only a modest reinforcement is observed for thenanocomposites below T_(g) (E′_(c)=5.1 GPa with 16.5% v/v whiskers,Supplementary FIG. 8). However, a dramatic effect is observed aboveT_(g), where E′ is increased from 1.0 MPa for the neat polymer matrix upto 814 MPa with 16.5% v/v whiskers (at 56° C.). The experimental dataabove T_(g) match well with the percolation model (FIG. 8), whichindicates that also in this series a percolating network of stronglyinteracting whiskers is formed. The nanocomposites demonstratesignificant swelling in both deionized water and ACSF. The solventuptake increases with increasing whisker content and temperature (FIG.9), lowers the T_(g) to below physiological temperature (19-23° C., FIG.10), and reduces E′_(c) dramatically. For example, the E′_(c) of a 16.5%v/v whisker nanocomposite above T_(g) is reduced from 814 MPa (dry) to10.8 MPa (water swollen; data are for 56 and 37° C., respectively; i.e.,16° C. above the respective T_(g)). As for the water-swollenPEO-EPI/whisker nanocomposites, the moduli of the wet PVAc/whiskernanocomposites are better described by the Halpin-Kardos than thepercolation model (FIG. 8), again indicative of decoupling of thestress-transferring nanofiber network upon introduction of water.

Exposure to brain tissue, simulated here by immersing the samples intoartificial cerebral spinal fluid (ACSF) and heating to a physiologicaltemperature of 37° C. at ˜2° C./min, leads to a pronounced reduction ofE′_(c). While the neat PVAc (dry E′_(c)=1.8 GPa at 25° C.) instantlysoftens under these conditions, the E′_(c) of the whisker-reinforcednanocomposites is reduced slowly over a period of 15 min. Thewhisker-reinforced nanocomposite displays a much higher dry E′_(c)=(4.2GPa at 25° C.) than the neat PVAc, but both materials reach nearlyidentical moduli upon immersion in ACSF at 37° C. (1.6 MPa).

In another set of experiments, the effect of the hydrophilicity of thematrix polymer on the dynamic mechanical properties of the nanocompositewas studied using polybutylmethacrylate (PBMA). PBMA, which is anamorphous polymer having T_(g) around 70° C., was dissolved in DMF (5%w/w) by stirring for two days. Nanocomposites were prepared by combiningthe desired amounts (to yield materials containing 0.7%-23% v/vwhiskers) of the colloidal whisker dispersion and polymer solution, andsolution-casting the resulting homogeneous mixture into Teflon® Petridishes. The dishes were placed into a vacuum oven (60° C., 15 mbar, 1week) to evaporate the solvent and dry the resulting films, before thematerial was compression-molded between spacers in a carver laboratorypress (90° C. at 0 psi for 2 min., followed by an increase of pressureto 3000 psi for 15 min.) to yield 50-500 μm thin nanocomposite films.PBMA tunicate whisker nanocomposites also showed a similar stimuliresponsive switching behavior but the contrast was less due to reducedwater uptake. For example, the E′_(c) of a 23% v/v whisker nanocompositeabove T_(g) was reduced from 1.1GPa (dry) to 99 MPa after being swollenin ACSF at 37° C. (In this case the E′_(c) data are given for 85° C. and65° C., respectively; i.e., ˜15° C. above the respective T_(g)). PBMA isrelatively less hydrophilic than PVAc and shows a T_(g) around 50° C.when plasticized. Hence, swelling at 37° C. (below the T_(g)) led to avery moderate water uptake (33%) and a significantly higher wet modulus(99 MPa). Swelling the same nanocomposite at 65° C. (i.e. 15° C. aboveT_(g)), instead of 37° C. as described above, led to higher water uptake(79%) and lower wet modulus (32 MPa). This illustrates the synergisticinteraction of thermal transition and swelling to effect the dynamicchange in modulus in these systems.

In another set of experiments, blends of PBMA and PVAc also showedsimilar switching behavior. Tunicate whisker nanocomposites withPVAc/PBMA blends prepared by a similar method as above displayed asystematic increase in aqueous swelling and decrease in wet modulus at37° C. with increasing PVAc content. Specifically, for a 12.2% v/vtunicate whisker PVAc/PBMA blend nanocomposite, as PVAc content wasincreased from 20% to 60%, swelling in ACSF at 37° C. increased from 35%to 60% and wet modulus at 37° C. dropped from 100 MPa to 24 MPa, whilethe dry modulus at 25° C. remained between 4 to 5 GPa.

In another study, use of a hydrophobic polymer such as polyethylene asmatrix showed a lesser contrast in modulus due to limited aqueousswelling. Under similar switching conditions, polyethylene tunicatewhisker nanocomposite prepared by a template approach such as describedby J. R. Capadona, O. van den Berg, L. Capadona, D. Tyler, S. Rowan, andC. Weder, Nature Nanotechnology 2, 765 (2007), herein incorporated byreference, displayed a contrast in modulus from 1.5 GPa (dry, 25° C.) to700 MPa (wet 37° C.) for 35 wt. % whiskers. In addition to water andACSF, softening of the same nanocomposite was also observed when swollenin N,N-dimethylformamide which is known to disperse the whiskers but notsolubilize the polymer. As will be evident to those skilled in the art,any solvent that disperses the whiskers (as demonstrated by O. van denBerg, J. R. Capadona, and C. Weder, Biomacromolecules 8, 1353 (2007)),but does not dissolve the matrix polymer, should serve as a sufficientchemical switching mediator.

In another study we also found that the aqueous swelling and wet modulusof the nanocomposites can be tailored by substituting in a controlledmanner the hydroxyl groups of cellulose with hydrophobic moieties.Esterification of cellulose with 2-Dodecen-1-yl succinic anhydride andincorporation of these modified whiskers in PVAc by solution casting asdescribed above significantly limits the aqueous swelling. In apreferred embodiment, PVAc nanocomposite with 12.2% v/v of thesemodified whiskers displayed a swelling of about 10-50% (ACSF 37° C., 1week) and a wet modulus at 37° C. of 15-40 MPa while the dry modulus at25° C. remained between 4-5 GPa.

In another study, we also found that the wet modulus of thenano-composites can be tailored by substituting, in a controlled manner,the hydroxyl groups of cellulose with carboxylic acid moieties.Oxidation of the primary hydroxyl groups and incorporation of thesemodified whiskers in PEO-EPI by solution casting, as described above,significantly decreases the modulus of the aqueous swollennanocomposites, by up to 10 fold. In a preferred embodiment, PEO-EPInano-composite with 18% v/v of these modified whiskers displayed a wetmodulus at 25° C. of 5.85 MPa while the dry modulus at −52° C. (−15° C.below T_(g)) remained between 8-9 GPa.

We also found the dynamic change in modulus of nanocomposites to beeffective using other sources of cellulose as reinforcement. Cellulosewhiskers derived from cotton with a lower aspect ratio of about 11 alsodisplayed a significant switching in modulus. In a preferred embodiment,a 12.2% v/v cotton whisker PVAc nanocomposite prepared as describedabove showed a limited swelling of about 20% (ACSF 37° C., 1 week) and aswitch in modulus from 4 GPa (dry, 25° C.) to 5 MPa (wet 37° C.).

In accordance with the patent statutes, the best mode and preferredembodiment have been set forth; the scope of the invention is notlimited thereto, but rather by the scope of the attached claims.

1. A method for inducing a modulus change in a polymer nanocomposite,comprising the steps of: providing a polymer nanocomposite comprising ananoparticle network incorporated into a host matrix polymer, whereinthe nanoparticle network is a substantially continuous three-dimensionalnetwork of substantially dispersed nanoparticles that exhibit at leastsome interactions among each other; and inducing a modulus change in thepolymer nanocomposite by exposing the polymer nanocomposite to astimulus that reduces interactions among the nanoparticles, wherein themodulus change exhibited is by a factor greater than 2.5.
 2. The methodaccording to claim 1, wherein said stimulus comprises a chemicalstimulus, an electrical stimulus, an optical stimulus, or a thermalstimulus, or a combination thereof.
 3. The method according to claim 1,wherein said stimulus is said chemical stimulus that reduces theinteractions among the nanoparticles by way of competitive binding. 4.The method according to claim 2, wherein the nanoparticles have particleto particle interaction due to hydrogen bonding, ionic charges,hydrophobic interactions or pi-pi stacking or a combination thereof, andwherein the nanoparticles have an aspect ratio (length/diameter) of 5 ormore.
 5. The method according to claim 2, wherein the nanoparticlescomprise nanofibers, nanotubes, nano-size platelet materials, or acombination thereof.
 6. The method according to claim 4, wherein thenanoparticles form a substantially percolating network prior to theinducing of the modulus change, and wherein the modulus change exhibitedis by a factor of 40 or more.
 7. The method according to claim 5,wherein the nanoparticles comprise nanofibers, and wherein thenanofibers are present in the polymer nanocomposite in an amount fromabout 3% to about 40% by volume based on the total volume of the firstmatrix polymer.
 8. The method according to claim 7, whereinnanoparticles are present in an amount from about 5% to about 30% byvolume based on the total volume of the first matrix polymer.
 9. Themethod according to claim 5, wherein the host matrix polymer comprisesan alkylene oxide polymer, an alkylene oxide copolymer, a vinyl aromaticpolymer, a vinyl aromatic copolymer, polyolefin polymer, a polyolefincopolymer, a diene polymer, a diene copolymer, a polyacrylate, anacrylate copolymer, a polyamide, a polyester polymer or a polyestercopolymer, or a combination thereof, and wherein inducing the moduluschange comprises exposing the polymer nanocomposite to a chemicalstimulus comprising a solvent, wherein the solvent is water,N,N-dimethyl formamide, dimethyl sulfoxide, N-methyl pyrrolidone, formicacid, or m-cresol, or a combination thereof.
 10. The method according toclaim 9, wherein the polymer is the ethylene oxide polymer, the ethyleneoxide copolymer, the polyacrylate, the acrylate copolymer, the polyesterpolymer, or the polyester copolymer, or a combination thereof, whereinthe nanoparticles have an aspect ratio (length/diameter) of 20 or more,and wherein the modulus change exhibited is by a factor of 5 or more.11. The method according to claim 10, wherein the modulus changeexhibited is by a factor of 10 or more, and wherein the nanoparticlescomprise tunicate whiskers.
 12. The method according to claim 9, whereininducing the modulus change further comprises increasing the temperatureof the polymer nanocomposite.
 13. A method for inducing a modulus changein a polymer nanocomposite, comprising the steps of: providing a polymernanocomposite comprising a nanoparticle network incorporated into a hostmatrix polymer, wherein the nanoparticle network is a substantiallycontinuous three-dimensional network of substantially dispersednanoparticles; and inducing a modulus change in the polymernanocomposite by exposing the polymer nanocomposite to at least acombination of chemical and temperature stimuli that reduce nanoparticleself interactions, wherein the modulus change exhibited is by a factorgreater than 2.5.
 14. The method according to claim 13, wherein themodulus change is by a factor of 5 or more, wherein the nanoparticlescomprise nanofibers, nanotubes, nano-size platelet materials, or acombination thereof, wherein the host matrix polymer comprises analkylene oxide polymer, an alkylene oxide copolymer, a vinyl aromaticpolymer, a vinyl aromatic copolymer, polyolefin polymer, a polyolefincopolymer, a diene polymer, a diene copolymer, a polyacrylate, anacrylate copolymer, a polyamide, a polyester polymer or a polyestercopolymer, or a combination thereof, wherein inducing the modulus changecomprises exposing the polymer nanocomposite to a chemical stimulus, andwherein the chemical stimulus comprises a solvent, wherein the solventis water, N,N-dimethyl formamide, dimethyl sulfoxide, N-methylpyrrolidone, formic acid, or m-cresol, or a combination thereof.
 15. Themethod according to claim 14, wherein the nanoparticles comprisenanofibers, and wherein the nanofibers are present in the polymernanocomposite in an amount from about 3% to about 40% by volume based onthe total volume of the first matrix polymer, wherein the nanoparticleshave an aspect ratio (length/diameter) of 20 or more, and wherein themodulus change exhibited is by a factor of 10 or more.
 16. A polymernanocomposite, comprising: a nanoparticle network incorporated into ahost matrix polymer, wherein the nanoparticle network is a formation ofa substantially three-dimensional network of substantially dispersednanoparticles, wherein in a first switched state, the composite has afirst modulus, and wherein in a second unswitched state, the compositehas a second modulus, wherein the first modulus is greater than thesecond modulus by a factor greater than 2.5.
 17. The nanocompositeaccording to claim 16, wherein the nanoparticles comprise nanofibers,and wherein the nanofibers are present in the polymer nanocomposite inan amount from about 3% to about 40% by volume based on the total volumeof the first matrix polymer.
 18. The nanocomposite according to claim17, wherein nanoparticles are present in an amount from about 5% toabout 30% by volume based on the total volume of the first matrixpolymer.
 19. The nanocomposite according to claim 18, wherein thenanoparticles have particle to particle interaction due to hydrogenbonding, ionic charges, hydrophobic interactions or pi-pi stacking, andwherein the nanoparticles have an aspect ratio (length/diameter) of 5 ormore.
 20. The nanocomposite according to claim 16, wherein thenanoparticles comprise nanofibers, nanotubes, nano-size plateletmaterials, or a combination thereof.
 21. The nanocomposite according toclaim 20, wherein the nanoparticles are in a form of a substantiallypercolating network in the first unswitched state.
 22. The nanocompositeaccording to claim 20, wherein the host matrix polymer comprises analkylene oxide polymer, an alkylene oxide copolymer, a vinyl aromaticpolymer, a vinyl aromatic copolymer, polyolefin polymer, a polyolefincopolymer, a diene polymer, a diene copolymer, a polyacrylate, anacrylate copolymer, a polyamide, a polyester polymer or a polyestercopolymer, or a combination thereof.
 23. The nanocomposite according toclaim 22, wherein the polymer is an ethylene oxide polymer, an ethyleneoxide copolymer, a polyester polymer, or a polyester copolymer, or acombination thereof, wherein the nanoparticles have an aspect ratio(length/diameter) of 20 or more, and wherein the modulus changeexhibited is by a factor of 5 or more.
 24. The nanocomposite accordingto claim 23, wherein the modulus change exhibited is by a factor of 10or more, and wherein the nanoparticles comprise tunicate whiskers. 25.The nanocomposite according to claim 16, wherein the nanoparticles havean aspect ratio (length/diameter) of 20 or more, wherein thenanoparticles are present in the polymer nanocomposite in an amount fromabout 3% to about 40% by volume based on the total volume of the firstmatrix polymer, wherein the nanoparticles comprise nanofibers, carbonnanotubes, nano-size platelet materials, or a combination thereof, andwherein the modulus change exhibited is by a factor of 40 or more.